Little progress has been made in the treatment of acute myeloid leukemia (AML) despite dose intensification of cytotoxic chemotherapy. An alternative approach to treating AML is the incorporation of pro-differentiation agents into standard chemotherapy regimens. In order to identify new AML differentiation agents, our laboratory developed a gene expression-based approach to small molecule screening. We identified gefitinib, an epidermal growth factor (EGFR) inhibitor, as an inducer of AML differentiation. EGFR is not expressed in the tested AML cell lines, thus precluding inhibition of this kinase as the mechanism of AML differentiation. Because multiple EGFR inhibitors induce the phenotype, we hypothesize that a shared off-target kinase is the target in AML differentiation. In order to identify candidate gefitinib targets of AML differentiation, we utilized proteomic and genetic approaches. Spleen tyrosine kinase (Syk) was identified as the top candidate. Syk is a nonreceptor tyrosine kinase, important in normal B-cell differentiation, and implicated in hematological malignancies. We hypothesize that Syk is a target for AML therapy and that loss of Syk will result in differentiation and/or cell death in AML. We confirmed with both pharmacological inhibition (R406) and genetic loss of Syk the induction of differentiation and/or death in a pilot study of AML cells. We now propose to more broadly test this hypothesis with the following Specific Aims. Specific Aim 1. Characterize the in vitro and in vivo effects of R406 in AML Specific Aim 2. Establish that Syk is the target of R406 activity in AML Specific Aim 3. Determine the downstream effectors of Syk in AML In Aim 1, we will determine the broad potential of Syk inhibition as an anti-AML therapy. We will measure the in vitro effects of a Syk inhibitor (R406) in a large panel of AML cells on differentiation, cell growth, and apoptosis. We will then extend testing to in vivo studies using primary human AML orthotopic models. Next, in Aim 2, we will confirm that Syk is the target of R406 activity with three parallel approaches: A PCR mutagenesis screen for Syk mutants that rescue the effects of R406 anti-AML activity;a genetic approach using RNA interference, and a pharmacological approach evaluating other small molecule inhibitors of Syk. In Aim 3, we will determine which proteins are critical downstream effectors of Syk signaling in AML using complementary approaches: biochemical, genetic, genomic, pharmacological, and proteomic. The Rigel Pharmaceutical compound, R788, the prodrug of R406, is already in Phase II testing and was recently demonstrated to have activity in autoimmune disease and lymphoma. With Phase I testing now complete and efficacy demonstrated for these diseases, we would be well positioned to rapidly bring R788 to clinical trial. These studies, within the five year time frame of this grant, will have immediate translational relevance and inform the development of a clinical trial testing Syk inhibition in patients with relapsed/refractory AML. PUBLIC HEALTH RELEVANCE: The studies in this proposal will validate the protein spleen tyrosine kinase (Syk) as a target for acute myeloid leukemia (AML) therapy. We will test pharmacological and genetic inhibition of Syk in AML cells in vitro and in vivo models of AML. Because there is already a drug with anti-Syk activity with demonstrated efficacy in lymphoma and autoimmune disease, we anticipate the rapid translation of these studies to a clinical trial testing Syk inhibition for patients with AML.